Seismic surveying is the practice of studying subterranean formations from reflections by those formations of acoustic waves. This includes imparting acoustic waves into a natural environment so that they may enter the earth and travel through the subterranean geological formations of interest. During their travels through the formations, certain features of the formations will reflect the waves back to the surface where they are recorded. The recorded reflections are then studied to ascertain information about those formations.
One type of seismic survey is the “marine” seismic survey. The term “marine” only indicates that the survey occurs in or on the water. It does not necessarily imply that the survey is occurring in a saltwater environment. While a marine seismic survey may occur in a saltwater environment, such as the ocean, it may also occur in brackish waters such as are found in bays, estuaries, and tidal swamps. They may even be conducted in wholly freshwaters such as are found in lakes, marshes, and bogs.
There are many kinds of seismic sources whose designs are typically, to some degree, tailored to the environment in which they are intended for use. Marine seismic surveys are frequently performed using what is called a “swept” source. The term “swept” comes from the operation of such sources, in which they “sweep” through a band of frequencies during the transmission of the seismic signal.
One recent development in marine seismic surveying is the acquisition of “humming” data, i.e. data from a signal generated from a “humming” source. “Humming” is using a non-impulsive controlled-frequency source that generates substantially all of its energy at a single frequency. Due to practical stability limitations the source may instead perform a controlled or uncontrolled drift within a narrow frequency range, typically staying within plus or minus one tenth of an octave around the nominal frequency. This is sometimes called “monochromatic” or “near monochromatic”, for example in U.S. application Ser. No. 13/327,524.
Humming acquisition may occur in several different ways. For example, stepped humming is a sequential humming acquisition in which a single source steps over a set of two or more discrete frequencies, one at a time. The time spent moving between frequencies should be very small compared to the time spent at each frequency. Another example, chord humming, is acquisition in which one or more sources simultaneously hum at differing, discrete frequencies. More information is available in U.S. application Ser. No. 13/327,524.
Another relatively recent development in seismic acquisition is “low frequency” acquisition. Seismic surveying historically has used frequencies in the range of 10-250 Hz for seismic signals because of their suitability in light of technical challenges inherent in seismic surveying. The term “low frequencies” is understood within this historical context, as frequencies below which getting sufficient signal to noise with conventional sources rapidly becomes more difficult as the frequency decreases (i.e. below about 6-8 Hz).
One example of a low frequency source that can sweep, or hum, or both sweep and hum, at low frequency is disclosed and claimed in U.S. patent application Ser. No. 12/995,763, filed Jun. 17, 2009. This particular source consists of a tunable mechanical resonator, which together with its control system, comprises a self-excited oscillator. The control system therein detects the velocity of the radiating piston and applies a drive force in the same direction as the detected velocity, causing the system to oscillate at or near its natural frequency. That frequency is controlled by varying the stiffness of a gas spring, so that the system can be caused either to oscillate at a single chosen frequency (i.e. “hum”) or over a continuous band of frequencies at some chosen rate (i.e. “sweep”). More information is available in U.S. application Ser. No. 12/995,763.
However, one issue with low frequency sources is that the frequency introduces problems that typical, conventional seismic frequencies do not. One way to address these problems is through accurate and precise control of the source's operation. For example, one type of control is known as frequency stability control, in which the operation of the source is controlled to help stabilize the frequency at which it emits the seismic signals.
Seismic sources, such as those presented in the aforementioned applications, are suitable for their intended purpose. However, the art is always receptive to improvements or alternative approaches, methods and configurations. The art will therefore well receive the seismic source described herein.